Global Patterns of Precipitation

Global precipitation patterns are largely determined by air masses and their movements, which in turn are produced by global air circulation patterns. Before taking a detailed look at global precipitation patterns, let’s look at Figure, which shows the general patterns expected for a hypothetical supercontinent that has most of the features of the Earth’s continents but is simplified. The map recognizes and defines five classes of annual precipitation: wet, humid, sub humid, semiarid, and arid.

A schematic diagram of annual precipitation over an idealized continent and adjoining seas.

Beginning with the equatorial zone, the figure shows a wet band stretching across the continent. This band is produced by convective precipitation over the equatorial lows near the intertropical convergence zone. Note that the wet band widens and is extended poleward into the tropical zone along the continent’s eastern coasts. This region is kept moist by the influence of the trade winds, which move warm, moist mT air masses and tropical cyclones westward onto the continental coast. Farther poleward, humid conditions continue along the east coasts into the midlatitude zones. In these regions, subtropical high-pressure cells tend to move mT air masses from the southeast onto the continent in the summer, whereas in winter, midlatitude cyclones bring cyclonic precipitation from the west.

In the arctic zone, shown on the continent as arctic desert, precipitation remains low because air temperatures are low, and only a small amount of moisture is contained in cold air. 

Another important feature of the hypothetical continent is the pattern of arid and semiarid regimes that stretches from tropical west coasts to subtropical and midlatitude continental interiors. In the tropical and subtropical latitudes, the arid pattern is produced by dry, subsiding air in persistent subtropical high-pressure cells. The aridity continues eastward and poleward into semiarid continental interiors, which remain relatively dry because they are far from source regions for moist air masses. Rain shadow effects provided by coastal mountain barriers are also important in maintaining inland aridity. 

Yet another obvious feature of the supercontinent is the pair of wet bands along the west coasts of the midlatitude and subarctic zones. These are produced by the eastward movement of moist mP air masses onto the continent, as driven by the prevailing westerlies. 

Global Precipitation Regions: 

1. Wet equatorial belt. 

This zone of heavy rainfall, over 2000 mm annually, straddles the Equator and includes the Amazon River Basin in South America, the Congo River Basin of equatorial Africa, much of the African coast from Nigeria west to Guinea, and the East Indies. In this zone, the warm temperatures and high-moisture content of the mE air masses favor abundant convective rainfall. At low latitudes, annual rainfall is heavy in the wet equatorial belt and trade-wind coasts. In contrast, tropical deserts are very dry.

2. Trade-wind coasts. 

Narrow coastal belts of high rainfall, 1500 to 2000 mm, extend from near the Equator to latitudes of about 25° to 30° N and S on the eastern sides of every continent or large island. Examples include the eastern coast of Brazil, Central America, Madagascar, and northeastern Australia. The rainfall of these coasts is supplied by moist mT air masses from warm oceans that are brought over the land by the trade winds and encounter coastal hills and mountains, producing heavy orographic rainfall.

3. Tropical deserts. 

In contrast to the wet equatorial belt are the zones of tropical deserts lying approximately on the Tropics of Cancer and Capricorn. These are hot, barren deserts with less than 250 mm of rainfall annually and, in many places, with less than 50 mm. They are located under the large, stationary subtropical cells of high pressure, in which the subsiding cT air mass is adiabatically warmed and dried.

4. Midlatitude deserts and steppes. 

Farther northward, in the interiors of Asia and North America between latitude 30° and latitude 50°, are great deserts, as well as vast expanses of semiarid grasslands known as steppes. Annual precipitation ranges from less than 100 mm in the driest areas to 500 mm in the moister steppes. Located in regions of prevailing westerly winds, these arid lands are far from sources of oceanic moisture and typically lie in rain shadows on the lee side of coastal mountains and highlands. In the southern hemisphere, the dry steppes of Patagonia, lying on the lee side of the Andean chain, are roughly the counterpart of the North American deserts and steppes.

5. Moist subtropical regions. 

On the southeastern sides of the continents of North America and Asia, in latitude 25° to 45° N, are the moist subtropical regions, with 1000 to 1500 mm of rainfall annually. Smaller areas of the same type are found in Uruguay, Argentina, and southeastern Australia. These regions are positioned on the moist western sides of the oceanic subtropical high-pressure circulations, which bring moist mT air masses from the tropical ocean onto the continent.

6. Midlatitude west coasts. 

Another wet location is on midlatitude west coasts of all continents and large islands lying between latitudes about 35° and 65° in the region of prevailing westerly winds. In these zones, abundant orographic precipitation occurs as a result of forced uplift of mP air masses. Where the coasts are mountainous, as in British Columbia, southern Chile, Scotland, and South Island of New Zealand, the annual precipitation is over 2000 mm. At higher latitudes, east and west coasts have higher precipitation, while continental interiors and arctic and polar regions are drier.

7. Arctic and polar deserts. 

A seventh precipitation region is formed by the arctic and polar deserts. Northward of the 60th parallel, annual precipitation is largely under 300 mm, except for the west-coast belts. Cold cP and cA air masses cannot contain much moisture, and, consequently, they do not yield large amounts of precipitation. 

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